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Detection of seismic quiescences before 1991 Uttarkashi (MW 6.8) and 1999 Chamoli MW (6.6) earthquakes and its implications for stress change sensor

Chingtham Prasanta, Sharma Babita

Acta Geophysica

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2022

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Vol. 70, no 2

509--523

EN

The statistically point process model known as epidemic-type aftershock sequence (ETAS) model is employed for systematically investigating the seismic quiescence or seismic anomalies around the focal regions of large/strong earthquakes for NW Himalaya. For this propose, the model predicted the expected occurrence rates of earthquakes by estimating the model parameters from the earthquake occurrences times using maximum likelihood method, has been used. Then the exhibited relative quiescence due to decreasing occurrence rates from the modeled ones can be identified by inspecting the abnormally downward deviated plot from the extended cumulative curve of the Residual Point Process (RPP) events. Examination of such RPP events in the whole time interval exhibits significant 1.5 years and 2.0 years of relative seismic quiescence before the strong 1991 Uttarkashi (MW 6.8) and 1999 Chamoli (MW 6.6) earthquakes, respectively. Considering the optimally oriented planes of Uttarkashi earthquake, the Coulomb stress changes (ΔCFS) have been investigated to check the rate of seismicity around the focal region of Chamoli earthquake. It has been found that ΔCFS of Uttarkashi earthquake exhibits stress shadow in or near the source zone of Chamoli earthquake and eventually decreases seismicity rates due to seismic quiescence in the source zone. On the other hand, the detected quiescence and activation relative to the predicted seismicity rate are consistent with the obtained Coulomb stress to depict the associated anomalies being sensitive enough to detect a slight stress change in the study region. Henceforth, the increased or decreased seismic activity due to seismic activation or quiescence is found to be consistent with the patterns of the Coulomb’s stress changes calculated on the ruptured fault planes of Uttarkashi earthquake. Hence, this ETAS model based on statistical technique can thus be incorporated with other sensitive geophysical instruments for identifying seismically quiet period not only in the seismic gaps, but also in its neighborhoods along the Himalayan range for mitigating seismic hazards due to impending great earthquakes.

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The seismogenic tectonic of Anchorage Mw7.0 Earthquake in Alaska, November, 30, 2018

Lei Dongning, Yang Gang

Acta Geophysica

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2020

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Vol. 68, no. 3

619--626

EN

The Mw7.0 Anchorage earthquake is most signifcant event since 1964 Mw9.2. Far away subduction zone trace, thus the seismogenic structure and failure plane are not clear and keep disputed. Based on published data, including faults, aftershock, focal mechanism solutions and source fault model and so on, we make comprehensive analysis and discussion. We think that this earthquake belongs to plate subduction event, without relationship with Castle Mountain fault and Border Ranges fault for seismogenic structure, only earthquake-controlling structure. Both faults are not responsible for the Anchorage earthquake. The actual seismogenic fault plane has a south-to-north trending and dips to east, which is corresponding to the nodal plane I.

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Fault modelling, seismic sequence evolution and stress transfer scenarios for the July 20, 2017 (MW 6.6) Kos–Gökova Gulf earthquake, SE Aegean

Sboras Sotirios, Lazos Ilias, Mouzakiotis Evaggelos, Karastathis Vassilios, Pavlides Spyros, Chatzipetros Alexandros

Acta Geophysica

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2020

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Vol. 68, no. 5

1245--1261

EN

The July 20, 2017, MW 6.6 Kos–Gökova Gulf earthquake occurred ofshore, near Bodrum of Turkey and Kos of Greece. It was one of the strongest in the broader area during the last many decades causing two deaths, many injuries and extensive damages. We investigated the evolution of the seismic sequence using seismological and geological tools. The aftershock sequence was relocated mainly in order to defne the geometry of the main seismic source, depicting a NNW-dipping fault plane. It also revealed signifcant clustering, associated with other nearby faults, and asymmetric spatio-temporal evolution. Along with morphotectonic analysis on Kos Island, and other published seismological information (e.g. focal mechanisms), we modelled the seismic source of the mainshock, as well as the one of the strongest aftershocks (August 8, MW 5.3). We applied the Coulomb failure criterion in order to investigate the efect of the mainshock on the strongest aftershock, and the rest of the sequence as well. Using the same method, we also investigated the stress changes of both strongest shocks for the prevailing E–W-trending normal faults in this area. Among other conclusions and implications, we deduce that the prevailing tectonic setting of the Gökova Gulf consists of roughly E–W-striking normal faults forming inner horsts and grabens.

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Ocena istotności wpływu transferu naprężeń statycznych wywołanych przemieszczeniem kosejsmicznym wstrząsów górniczych na proces sejsmogeniczny w kopalniach

Orlecka-Sikora B.

Geologia / Akademia Górniczo-Hutnicza im. Stanisława Staszica w Krakowie

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2009

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T. 35, z. 2/1

507-514

EN

We investigated an effect of static stress transfer for the mining-induced seismicity from Rudna Mine. We consider events of energy over 10/5J, which occurred in Rudna Mine from 1993-2006. We examine the possible triggering checking correlation between event locations and the stress-increased zones. We find that more than 50% of the analyzed seismic events occurred in areas where stress was enhanced due to the occurrence of previous events. Statistical test proved that for stress changes from 0.02 bar static stress triggering in Rudna Mine exists and this effect is statistically significant at the 95% confidence level.

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A study of the interaction among mining-induced seismic events in the Legnica-Głogów Copper District, Poland

Orlecka-Sikora B., Papadimitriou E., Kwiatek G.

Acta Geophysica

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2009

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Vol. 57, no. 2

413-434

EN

We applied the Coulomb stress transfer technique to investigate interactions among seismic events induced by mining works in the Rudna mine in the Legnica-Glogów Copper District in Poland. We considered events with energy greater than 10⁵ J from the period 1993-1999. We examined the influence of the cumulative static stress changes (ΔCFF) due to previous events on the generation of subsequent ones. The results indicate that in many cases strong mining tremors produce changes in the state of stress of a sufficient magnitude to influence subsequent events. The location of over 60% of events is consistent with stress-enhanced areas where the values of ΔCFF were above 0.01 MPa. For most of the events located inside areas of a calculated negative ΔCFF, their modelled rupture zone was partially located inside stress enhanced area, providing thus additional evidence for possible triggering at the nucleation point.